Open Data supplied by Natural Environment Research Council (NERC)

Neil Brown MK3 CTD

The Neil Brown MK3 conductivity-temperature-depth (CTD) profiler consists of an integral unit containing pressure, temperature and conductivity sensors with an optional dissolved oxygen sensor in a pressure-hardened casing. The most widely used variant in the 1980s and 1990s was the MK3B. An upgrade to this, the MK3C, was developed to meet the requirements of the WOCE project.

The MK3C includes a low hysteresis, titanium strain gauge pressure transducer. The transducer temperature is measured separately, allowing correction for the effects of temperature on pressure measurements. The MK3C conductivity cell features a free flow, internal field design that eliminates ducted pumping and is not affected by external metallic objects such as guard cages and external sensors.

Additional optional sensors include pH and a pressure-temperature fluorometer. The instrument is no longer in production, but is supported (repair and calibration) by General Oceanics.

RRS Discovery Cruise D233 CTD Instrumentation

The CTD profiles were taken with a WOCE Standard Neil Brown Systems MkIIIc CTD (DEEP04), with a FSI 24 bottle rosette.

Sensor

Manufacturer/Model

Serial Number

Last calibration date

Comments

CTD

Neil Brown MkIIIc

DEEP04

-

Incorporating an oxygen sensor

Fluorometer

Chelsea Instruments Aquatracka

88/2360/108

-

-

Transmissometer

Sea Tech transmissometer (1 m pathlength)

161/2642/003

-

Cruise reported stated this to be a Chelsea Instruments Transmissometer

Altimeter

Simrad

200 m range

-

-

LADCP

RDI (150 kHz) - 30 degree beam

-

-

-

Reversing thermometers

SIS

T401

-

Failed after stations 13415 and 13416

T989

-

Lost at station 13462

T995

-

-

T714

-

Failed after stations 13415 and 13416

Reversing pressure meters

SIS

P6075

-

Sensor failed on station 13440

P6394

-

-

P6132

-

Lost at station 13462

Aquatracka fluorometer

The Chelsea Instruments Aquatracka is a logarithmic response fluorometer. It uses a pulsed (5.5 Hz) xenon light source discharging between 320 and 800 nm through a blue filter with a peak transmission of 420 nm and a bandwidth at half maximum of 100 nm. A red filter with sharp cut off, 10% transmission at 664 nm and 678 nm, is used to pass chlorophyll-a fluorescence to the sample photodiode.

The instrument may be deployed either in a through-flow tank, on a CTD frame or moored with a data logging package.

RRS Discovery Cruise D233 CTD Processing

Introduction

Originator's Data Processing

Sampling Strategy

A total of 140 CTD stations (13414-13553) were completed during D233. Lowering rates for the CTD package were generally in the range 0.5-1.0 ms -1 but could be up to 1.5 ms -1 . Water samples were drawn on casts for the calibration of the instrumentation.

Data Acquisition and Processing

Data were captured by the SOC-DAPS (Southampton Oceanography Centre - Data Acquisition and Processing System) software. The DAPS software checks for pressure jumps and subsequently produces 1 sec averages of the raw 25 Hz data. Two ASCII files are generated for each cast, one for CTD profile data, the other for bottle firing data. Post processing of these output files was conducted in the PSTAR environment using the PEXEC suite of programs.

During D233, the CTD data were also logged simultaneously by the RVS Level A system and used in a backup capacity.

Field calibrations

A summary of the CTD calibrations applied to instrumentation are presented below. For further details please refer to the D233 Cruise Report .

Temperature

Raw temperatures were scaled according to:

T raw = 0.0005 T raw

then calibrated using the following equation (obtained from laboratory calibrations in July 1996):

T = 0.13079 + 0.999314 T raw

Due to a lag between the conductivity and temperature sensor measurements the time rate of change of temperature was used to "speed up" the temperature measurements according to:

T = T + τ δ T / δ t

where the rate of change of temperature is determined over a one second interval. The time constant, τ = 0.25, was used for D233.

Post-cruise calibration coefficients (obtained in October 2008) were subsequently compared with those documented above. No modifications were deemed necessary from these investigations.

Pressure

Raw pressure measurements were first scaled according to:

P raw = 0.1 P raw

then calibrated using the calibration (obtained from laboratory calibrations in July 1996):

P = -36.685 + 1.07333 P raw

Following laboratory calibration, no further corrections were deemed necessary for temperature dependence or pressure hysteresis.

Post-cruise calibration coefficients (obtained in October 2008) were subsequently compared with those documented above. As with temperature, no further changes were required.

Conductivity/Salinity

Raw conductivities were scaled according to:

C raw = 0.001 C raw

then calibrated using the following equation:

C = -0.015 + 0.96743 C raw

The coefficients above were obtained from comparison of CTD data with bottle samples from all water depths from the first seven casts. Additional small offsets were added to the correction periodically and applied to groups of stations. These offsets were obtained following examination of subsequent deep water (in excess of 2000 dbar) sample data. The table below records these additional corrections:

Station Number

Correction

13414-13415

0.0000

13416

0.0014

13417-13420

0.0000

13421-13422

-0.0010

13423-13424

-0.0019

13425-13428

-0.0027

13429-13436

-0.0035

13437-13442

-0.0044

13443-13456

-0.0057

13457-13461

-0.0043

13462-13474

-0.0062

13475-13484

-0.0067

13485

-0.0020

13486

0.0000

13487-13488

0.0030

13489-13494

0.0000

13495-13500

0.0030

13501-13504

0.0013

13505-13516

0.0000

13517-13522

-0.0038

13523-13546

-0.0085

13547-13553

-0.0070

CTD salinity was subsequently generated from the conductivity, C, via a PEXEC program.

Further details regarding the calibration can be found in the Cruise Report (Smythe-Wright, 1999).

Fluorometer

Fluorescence was converted to voltages using the CTD's voltage digitiser calibration supplied by Ocean Scientific International (OSI):

fvolts = -5.656 + 1.7267e -4 f raw + -2.244e -12 (f raw ) 2

Conversion of the fluorescence to chlorophyll concentration was subsequently carried out. Details of this calibration are not held by BODC.

Oxygen

A best fit of downcast CTD oxygen to measured oxygen samples (based on the model of Owens and R.C. Millard, 1985) was obtained and applied to each station. No CTD oxygen data were collected between stations 13415 and 13417.

Post-cruise investigations

A mismatch between the downcast and upcast CTD data from D233 was discovered during further analysis and is an issue that affected other cruises from this period. The problem is not believed to be caused by pressure hysteresis, but is probably a pressure or temperature effect on the temperature sensor or some electronic component of the CTD. With this in mind, 2 dbar binned versions of the upcast CTD (sorted on pressure) were generated as the preferred CTD dataset from D233. These data have been calibrated exactly to the water bottle samples and so have the most accurate salinity.

BODC Processing

Reformatting

The pressure-binned upcast PSTAR files were supplied to BODC for archiving. These were accompanied with corresponding downcast PSTAR files, also pressure-binned to 2 dbar. The latter contained the calibrated oxygen data, since the upcast oxygen suffered from hysteresis.

The upcast data (excluding oxygen) were merged with the downcast oxygen data (linked using common pressures) to produce a single BODC QXF file per cast. No further calibrations were applied to the data received by BODC. A number of derived channels were generated by BODC: potential temperature, sigma-theta and oxygen saturation. These were all calculated from the data streams merged into the QXF files, using established algorithms (for further details please refer to the parameter section of this report).

The following table summarises the mapping of originator variables to BODC parameter codes:

Originator's variable

Units

Description

BODC Parameter code

Units

Comments

press

dbar

Pressure exerted by the water body

PRESPR01

dbar

Calibrated by originator

temp

°C

Temperature of the water body

TEMPST01

°C

Calibrated by originator

salin

-

Practical salinity of the water body

PSALST01

-

Calibrated against bottle data by originator

oxygen

µmol/l

Dissolved oxygen

DOXYPR01

-

Calibrated against bottle data by originator

fluor

mg m -3

Concentration of chlorophyll-a

CPHLPR01

mg m -3

Calibrated against bottle data by originator

trans

m -1

Transmittance

-

-

Not preserved in BODC files - uncertainty over source data and applied calibrations

Quality Control

The reformatted data were visualised using the in-house EDSERPLO software. Suspect data were marked by adding an appropriate quality control flag, whilst missing data were marked by setting the data to an appropriate absent data value and assigning a quality control flag.

Banking

Quality controlled data were ingested into the National Oceanographic Data Base (NODB), where they are supported by comprehensive metadata. During this stage a number of inaccuracies were identified in the CTD Station logsheets that appear in the cruise report. These are noted below in their entirety and corrected for in BODC metadata, where necessary:

World Ocean Circulation Experiment (WOCE)

The World Ocean Circulation Experiment (WOCE) was a major international experiment which made measurements and undertook modelling studies of the deep oceans in order to provide a much improved understanding of the role of ocean circulation in changing and ameliorating the Earth's climate.

WOCE had two major goals:

Goal 1. To develop models to predict climate and to collect the data necessary to test them.

Goal 2. To determine the representativeness of the Goal 1 observations and to deduce cost effective means of determining long-term changes in ocean circulation.

UK WOCE

The UK made a substantial contribution to the international World Ocean Circulation Experiment (WOCE) project by focusing on two important regions:

VIVALDI, a seven year programme of seasonally repeated surveys to study the upper ocean.

Long-term observations of ocean climate in the North West Approaches.

Satellite ocean surface topography, temperature and wind data were merged with in situ observations and models to create a complete description of ocean circulation, eddy motion and the way the ocean is driven by the atmosphere.

The surveys were forerunners to the international Global Ocean Observing System (GOOS). GOOS was later established to monitor annual to decadal changes in ocean circulation and heat storage which are vital in the prediction of climate change.